1. Field of the Invention
The present invention relates to a vehicle-mounted power supply system capable of efficiently monitoring the state of charge of a rechargeable storage battery when the vehicle is parked.
2. Description of the Related Art
A vehicle-mounted rechargeable storage battery is monitored in a number of ways. In one method, when the engine is started the initial remaining capacity of the battery is calculated from its voltage-current characteristic and subsequently the charge/discharge current of the battery is integrated to determine its remaining capacity, as described in Japanese Patent No. 2910184. Another known method, when the automotive generator is not operating when the vehicle is parked, the voltage of the battery is monitored and its charge-discharge current is integrated. The remaining capacity of the battery is calculated from the monitored voltage and the integrated current value, as described in Japanese Patent Publication No. 2003-68369.
However, in the known methods current sensor and electronic monitoring circuitry need to operate at all times to monitor the battery even though the automotive generator is not working. Under such conditions, the current drained from the current sensor and monitoring circuitry represents the dark current of the battery and dissipates its storage power. As a result, no practical systems have been developed so far that are capable of monitoring the vehicle-mounted storage battery when the engine is stopped. Thus, prolonged parking of a vehicle could result in a storage battery having almost discharged its energy with no remaining power to restart the engine.